1. a Field of the Invention
Broadly speaking, this invention relates to filtration. More particularly, in a preferred embodiment, this invention relates to the filtration of noxious and offensive gases, particulate material and vapors, from the atmosphere.
2. B Description of the Prior Art
The rapid increase in the death rate due to lung cancer, emphysema, bronchitis and similar respiratory diseases has been traced to the growing amount of pollution in the air we breath, including the so-called "personal air-pollution" caused by cigarette smoking. The number of pollutants in the atmosphere are too numerous to list in detail but some of the principal components comprise carbon monoxide and lead from automobile exhausts; sulphur dioxide and soot from the burning of fossil fuel; and, in the case of cigarettes, nicotine, acrolein, phenol, tars, organic acids such as alcohol, acetic acid, ketones, esters, and other hydrocarbons and carbon dioxide, carbon monoxide and oxides of nitrogen.
Medical research has proved beyond doubt that many of the above-described pollutants are carcinogenic and some, such as acrolein (CH.sub.2 :CH.CHO), are ciliastatic, that is to say they paralyze the cilia which line the trachea and bronchioles. Normally, these whiplike appendages beat to and fro, in rhythmic fashion, and by this action carry foreign bodies up and out of the respiratory tract, thereby preventing foreign particulate matter from accumulating in the respiratory system. When the activity of the cilia is inhibited by acrolein or similar pollutants, harmful particulate material, including in the case of cigarette smoking any tar particles contained in the smoke, easily pass by the paralyzed cilia and collect in the lungs.
In view of the pressing health problem, vast research projects have been conducted, particularly in the cigarette industry, in an attempt to find a filter which will attenuate, or better still, eliminate most of these harmful elements. Unfortunately, the results to date have not been spectacular. While the smoker of a commercially manufactured filter cigarette is undoubtedly better off than if he were smoking a non-filtered cigarette, he nevertheless, runs a significant risk of contracting bronchitis or emphysema or other more dangerous illnesses.
Commercially available cigarette filters typically comprise a solid absorbent, such as activated charcoal or silica gel, sandwiched between a pair of cellulose acetate filter plugs. This type of filter removes from the tobacco smoke a proportion of the liquid or semi-liquid droplets passing through it. This is accomplished by a combination of diffusional, impactive, and direct collision of the droplets with the filter fibers. Upon collision, the droplets are retained on the fibers by the surface attraction between the extremely small particles and the relatively large fiber. Such fibrous filters are, however, not particularly effective for removing vaporized components from the smoke stream by the processes of physical and chemical adsorption. The smooth and non-porous nature of the commonly used fibrous filtering materials, while effective in capturing tobacco smoke droplets, does not present a sufficient surface area to effectively adsorb gaseous molecules. It has been calculated that an ordinary cellulose acetate cigarette filter plug has a surface area ranging from 1,000 to 10,000 square centimeters per gram of material. This affords insufficient surface area for effective adsorption of gaseous molemules, and has no significant selectivity. In some instances, where a vaporized material is sufficiently soluble in the fibrous material so that its surface concentration is rapidly depleted, a significant removal can be achieved by the process of absorption. An example of such a material present in tobacco smoke is phenol, which has a pronounced solubility in cellulose acetate filtering material. Cellulose acetate filters allow both hydrogen cyanide and hydrogen sulfide to pass through in undesirably high concentration.
In attempts to improve the adsorptive properties of tobacco smoke filters, various treatments of ordinary filtering material and various new fibrous filtering materials have been proposed. Among the materials and treatments proposed, a number of well-known adsorbants such as activated charcoal, alumina, natural and synthetic clays and silica gel have been proposed as additives to tobacco smoke filters. These materials are classified as absorbents and are used in gas-stream treatment because they possess in common the characteristic that their specific surface area exceeds a million square centimeters per gram of material. A good rule of gas adsorbent quality activated carbon, for example, has a specific surface area in excess of 5 million square centimeters per gram. In general, the preferred method heretofore used of including these adsorbents in tobacco filters has been by dusting, spraying, tumbling, slurrying, or otherwise incorporating the finely-divided adsorbent into the fibrous material which forms the filter or a part thereof.
Such adsorbent-containing tobacco smoke filters are only partially effective for several reasons. One is that the adsorbent is often rendered ineffective by the incorporation process and subsequent handling in the cigarette manufacturing and distribution process, in that water, plasticizing agents, glues, adhesives and volatile flavoring materials, in prolonged close contact with the finely-divided adsorbent, partially or completely utilize the adsorbent surface, thus decreasing its capacity to adsorb gaseous molecules from the smoke stream.
Another reason is that when adsorbent and fibrous filtering materials are intimately mixed, condensed droplets tend to be deposited in such a filter on or near an adsorbent particle, with the result that relatively nonvolatile smoke constituents quickly permeate the adsorption and thus reduce its capacity for adsorption of gaseous molecules.
Further disadvantages of such known adsorbent filters are that the inclusion of considerable quantities of adsorbent in a tobacco smoke filter has both a marked effect upon the draw resistance of the smoking article, and upon the taste of the smoke stream. A powdered of finely divided adsorbent dispersed in a matrix of fibrous filtering elements impedes the flow of smoke through the filter and requires that the smoker apply additional suction to withdraw his normal amount of smoke. If enough adsorbent is included to remove considerable quantities of deleterious materials, the draw resistance is well above a desirable and comfortable level.
On the other hand, segregation of the adsorbent into a portion of the filter separated from the fibrous portion thereof, has pronounced deleterious effects on the taste of the smoke. When adsorbents such as charcoal, alumina, or silica gel are incorporated in a cigarette filter (segregated from the cellulose acetate filter) in sufficient quantities to considerably reduce the levels of gaseous materials, it is found that the taste of the smoke stream is far from pleasing to experienced smoke tasters. Although some irritating factors are reduced, the smoke is found to have an astringent, bitter, drying taste. Other mineral adsorbents have a similar overall effect, with minor modification in taste and aroma depending on the particular adsorbent. The general result is that a pleasing smoke is obtained only when the amount of adsorbent is reduced to the point where it is well in minor proportion to the fibrous filtering agent. When present at such diminished levels, it is insufficiently effective in removing undesirable smoke constituents.
The problem then is to find a filter which does not rely on adsorption alone for its filtering action but which, nevertheless, is highly efficient in removing particulate, vapor and gaseous pollutants from the atmosphere. The filter should have application to the control of industrial pollution in general, and to the automobile industry in particular, and should also be of use in the cigarette industry. In this latter regard, the filter should be inexpensive, tasteless and should not add significantly to the amount of draw required from the cigarette.